A process for producing an unsaturated fatty acid from an olefin by way of an unsaturated aldehyde is a typical process of catalytic vapor phase oxidation. To perform partial oxidation of olefins, composite oxides containing molybdenum and bismuth, molybdenum and vanadium, or mixtures thereof are used as catalysts. Particular examples of such catalytic vapor phase oxidation include a process of producing (meth)acrylic acid by the oxidation of propylene or isobutylene by way of (meth)acrolein, a process of producing phthalic anhydride by the oxidation of naphthalene or orthoxylene, and a process of producing maleic anhydride by the partial oxidation of benzene, butylene or butadiene.
Generally, (meth)acrylic acid, a final product, is produced from at least one reaction material selected from the group consisting of propylene, propane, isobutylene, t-butyl alcohol or methyl-t-butyl ether (referred to as ‘propylene or the like’, hereinafter) by a two-step process of vapor phase catalytic partial oxidation. More particularly, in the first step, propylene or the like is oxidized by oxygen, inert gas for dilution, water steam and a certain amount of a catalyst, so as to produce (meth)acrolein as a main product. Then, in the second step, the (meth)acrolein is oxidized by oxygen, inert gas for dilution, water steam and a certain amount of a catalyst, so as to produce (meth)acrylic acid. The catalyst used in the first step is a Mo—Bi-based multinary metal oxide, which oxidizes propylene or the like to produce (meth)acrolein as a main product. Also, some acrolein is continuously oxidized on the same catalyst to partially produce (meth)acrylic acid. The catalyst used in the second step is a Mo—V-based multinary metal oxide, which mainly oxidizes (meth)acrolein in the mixed gas containing the (meth)acrolein produced from the first step to produce (meth)acrylic acid as a main product.
A reactor for performing the aforementioned process is provided either in such a manner that both the two-steps can be performed in one system, or in such a manner that the two steps can be performed in different systems.
As mentioned hereinbefore, the first-step catalyst involved in vapor phase partial oxidation using propylene or the like as a starting material is a multinary metal oxide, with which (meth)acrolein is produced as a main product and at most 10% of (meth)acrylic acid is produced.
As disclosed in Japanese Laid-Open Patent No. Hei8-3093, a conventional first-step catalyst is a composite oxide represented by the formula of MOa—Bib—Fec—Ad—Be—Cf—Dg—Ox, wherein Mo, Bi and Fe represent molybdenum, bismuth and iron, respectively; A is nickel and/or cobalt; B is at least one element selected from the group consisting of manganese, zinc, calcium, magnesium, tin and lead; C is at least one element selected from the group consisting of phosphorus, boron, arsenic, Group 6B elements in the Periodic Table, tungsten, antimony and silicon; D is at least one element selected from the group consisting of potassium, rubidium, cesium and thallium; each of a, b, c, e, f and g is a number satisfying the conditions of 0<b≦10, 0<c≦10, 1≦d≦10, 0≦e≦10, 0≦f≦20, and 0<g≦2, when a=12; and x is a value defined by the oxidation state of each element. When vapor phase catalytic oxidation of propylene is performed with molecular oxygen by using the above first-step catalyst and by operating the first-step catalyst layer at a temperature of 325° C., acrolein is produced with a yield of 81.3% and acrylic acid is produced with a yield of 11%. In other words, acrylic acid content is low in the reaction product obtained by using the first-step catalyst.
Meanwhile, Japanese Laid-Open Patent No. Hei5-293389 discloses a catalyst represented by the formula of MoaBibFecAdXeYfZgSihOi, wherein Mo, Bi, Fe, Si and O represent molybdenum, bismuth, iron, silicon and oxygen, respectively; A is at least one element selected from the group consisting of cobalt and nickel; X is at least one element selected from the group consisting of magnesium, zinc, manganese, calcium, chrome, niobium, silver, barium, tin, tantalum and lead; Y is at least one element selected from the group consisting of phosphorus, boron, sulfur, selenium, Group 6B elements in the Periodic Table, cerium, tungsten, antimony and titanium; Z is at least one element selected from the group consisting of lithium, sodium, potassium, rubidium, cesium and thallium; and each of a, b, c, d, e, f, g, h and i represents the atomic ratio of each element, with the proviso that when a=12, b=0.01˜3, c=0.01˜5, d=1˜12, e=0˜6, f=0˜5, g=0.001˜1, h=0˜20, and i is the oxygen atom number needed to satisfy the atomic valence of each element. When vapor phase catalytic oxidation of propylene is performed by using the above first-step catalyst to produce acrolein and acrylic acid, acrylic acid is produced with a yield of 6.2 mole % under a propylene conversion ratio of 99.1 mole % and an acrolein selectivity of 89.6 mole %. In other words, acrylic acid content is still low in the reaction product obtained by using the first-step catalyst.
In a process for producing (meth)acrylic acid, the temperature of the second-step catalyst layer varies depending on the selectivity of (meth)acrolein and (meth)acrylic acid (i.e. the first-step catalytic reaction product) and the amount of (meth)acrolein unreacted in the second-step catalytic reaction. The second-step catalyst layer is operated in such a manner that unreacted (meth)acrolein can be minimized. When (meth)acrolein selectivity is high in the first-step catalytic reaction product, the second-step catalyst layer is subjected to an increased load and concentration, resulting in an increase in reaction temperature and degradation of the lifetime of the catalyst. Additionally, when a concentration of unreacted (meth)acrolein is increased due to the degradation in catalytic activity, a waste gas incineration system (WGIS) may be overloaded, resulting in degradation of the lifetime of a waste gas treating catalyst.